Project Summary/Abstract Because activity-dependent plasticity is ubiquitous in the CNS, brain stimulation may have long-term effects on areas to which the stimulated area connects. These effects have received little attention. Nevertheless, recent appreciation of the long-term role of cortex in shaping spinal cord pathways suggests that the long-term spinal effects of cortical stimulation are likely to be substantial. In fact, weak electrical cortical stimulation (ECS) of rat sensorimotor cortex has lasting spinal effects. Three months after ECS ends, GABA receptors in spinal motoneurons (MNs) remain decreased and the H-re?ex (analog of the spinal stretch re?ex) remains increased. Furthermore, when incomplete spinal cord injury has impaired locomotion, ECS can improve locomotion. Our goal is to learn how ECS causes these spinal effects and characterize them physiologically, anatomically, and genomically, with emphasis on their clinical implications. Based on work to date, our core hypothesis is that: ECS excites GABAergic spinal interneurons (INs) that contact soleus MNs; the increased GABAergic input decreases GABA receptors and thus increases the H-re?ex; and key gene activations and/or post-translational events cause the transient increases in GABAergic INs and terminals, the persistent GABA receptor decrease, and thus the persistent H-re?ex increase. At the same time, initial data indicate that glutaminergic and cholinergic INs, terminals, and/or receptors also change; other spinal re?exes are probably affected as well. To assess our core hypothesis and illuminate the wider ECS effects, we have two aims. The ?rst aim is to determine how ECS parameters affect its impact on the spinal cord and how ECS in?uence reaches the spinal MN. We will model and test the effects of clinically-relevant epidural ECS frequencies and inter-electrode spacing on the direction and magnitude of spinal effects. We will use a chemogenetic method (DREADDs) to assess the roles of speci?c spinal INs, and tract-tracing methods to assess the role of the corticospinal tract (CST) and/or other descending pathways. The second aim is to characterize the short- and long-term spinal effects of ECS on physiological, anatomical, and genomic levels. These studies will: examine ECS impact on MN properties and major spinal re?ex pathways; explore ECS impact on GABAergic and other spinal INs and their terminals and receptors on different MN pools; and analyze the genomic mechanisms of the spinal functional and structural changes caused by ECS. In summary, this proposal takes advantage of the relative simplicity of the spinal cord and a well-de?ned experimental model to explore the spinal effects of ECS on the physiological, structural, and genomic levels. By characterizing the nature and mechanisms of the spinal plasticity caused by ECS, the results should give new insight into its wider effects, and into how the cortex modi?es the spinal cord throughout life. Furthermore, the results should guide development of ECS protocols to further explore these remote effects, and ECS protocols to induce bene?cial plasticity that enhances functional recovery after CNS trauma or disease.